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Microstructure evolution of vacuum tempered low carbon microalloyed steel

With the development of low-carbon microalloying and controlled rolling and controlled cooling technology, the problem of residual stress in high-strength steel has become increasingly prominent. In order to obtain high-strength steel products with excellent properties, the method of vacuum tempering after rolling is often used to control the steel structure and residual stress. However, due to the lack of understanding of the structure transformation law and residual stress control mechanism in the vacuum tempering process, in actual production, the internal stress of the material is generally reduced by increasing the vacuum tempering temperature and prolonging the vacuum tempering time, which will inevitably lead to production. Cost increases and steel grade performance decreases. Therefore, it is necessary to conduct in-depth research on the microstructure evolution of low-carbon microalloyed high-strength steel during vacuum tempering to solve the problem of residual stress in the steel.

1 Materials and methods

The steel used in this study is hot-rolled automobile beam steel 700L, the thickness specification is 16 mm, and the chemical composition is shown in Table 1.

A cylindrical sample with a size of 4 mm × 10 mm and a circular sample with a size of 5 mm × 0.2 mm were cut from the middle of the billet, and the thermal expansion experiments and differential scanning calorimetry experiments were carried out under continuous heating conditions, respectively. The non-isothermal tempering thermal expansion (DII) experiment was carried out on a TADIL805L thermal expansion phase change instrument with a resolution of 50 nm. The sample was heated to 700 °C at a heating rate of 2 °C/min, and then cooled to room temperature at 50 °C/min. The first vacuum tempering cycle was completed; then, the second vacuum tempering experiment was carried out with the same heating and cooling rate and target temperature, and the length change of the sample during the heating and cooling process was recorded. Differential scanning calorimetry (DSC) experiments were carried out on a NetzschSTA449C thermal analyzer in an Ar atmosphere. The sample was heated to 700 °C at a heating rate of 2 °C/min, and then cooled to room temperature at a rate of 50 °C/min to complete the first tempering cycle. Then repeat the above operation for the second tempering cycle, and record each vacuum tempering cycle. Heat flow changes in stages. The non-isothermal double tempering cycle method was used in this study, with the aim of adding a second non-isothermal tempering step as a baseline to reveal the expansion and heat flow changes of the test steels at different vacuum tempering stages.

Several samples with a size of 10 mm × 10 mm × 2 mm were cut from the hot-rolled billet by wire cutting, and were kept in a box furnace at 200, 300, 400, 500, 550, 600, 650, and 700 °C for 1 h for vacuum tempering treatment, and air-cooled to room temperature. After grinding, polishing and etching with 4% nitric acid alcohol solution, the microstructure of the steel samples was observed by Zeiss Axioplan2 optical microscope (OM) and FEI Nova 400 Nano field emission scanning electron microscope (SEM). The Vickers hardness of each steel sample was measured using a MICRO-586 indenter under a Vickers hardness tester, with a load of 4.9 N and a loading time of 10 s, and the average value of the six measurements was taken as the hardness of the sample (HVo. Cut slices with a size of 10 mm × 10 mm × 0.3 mm from the heat-treated samples, grind the samples with 400, 800, 1200, and 2000 sandpapers in turn until the thickness is less than 80 μm, and then punch the samples into a thickness of 3 mm with a punch. The small round piece was thinned by electrochemical polishing method, the electrolyte was 4% perchloric acid and ethanol solution, the voltage was 50V, and the steel sample was analyzed by JOELJSM 2100F 200kv field emission transmission electron microscope and the equipped energy dispersive spectrometer (EDS). The morphology, distribution and composition of the carbides were characterized.

2 Results and Analysis

DIL curve and DSC curve

At a heating rate of 2 °C/min, the results of the non-isothermal double-tempering expansion and non-isothermal double-tempering differential thermal experiments of the test steel are shown in Figure 1. Figure 1(a) shows the variation of the thermal expansion difference (ΔLi – ΔLz) of the test steels with the temperature for the first tempering, the second tempering and the two tempering steps. The amount of expansion caused by the transformation of the microstructure is small. In order to better reflect the change of the thermal expansion curve at each temperature stage, the first-order derivation of the DIL curve with respect to temperature is carried out, and the result is shown in Fig. 1(b). It can be clearly observed from Figure 1(b) that during the heating process, in the temperature ranges of 50-200 ℃, 200-30o ℃, 250-450 ℃, 450-580 ℃, and 580-650 ℃, respectively, after derivation There are obvious peaks and troughs in the DII curve of , indicating that the material expansion changes significantly in these temperature ranges, which must be accompanied by the occurrence of microstructure transformation. In addition, obvious exothermic peaks can be observed from the baseline-subtracted DSC curve shown in Fig. 1(d), and the temperature range is similar to that shown in Fig. 1(b). Therefore, these exothermic peaks are also considered to be related to each stages of organizational transformation. Accordingly, the vacuum tempering process of the test steel is divided into five stages: the first stage (50-200 ℃), the second stage (200-300 ℃), the third stage (250-450 ℃), the fourth stage (450 ~ 580 ℃), V stage (580 ~ 650 ℃), the thermal expansion and expansion rate of the test steel in each vacuum tempering stage are listed in Table 2.

The microstructure transformation of steel in the process of vacuum tempering is controlled by carbon diffusion, and the degree and speed of transformation depend on the diffusion capacity of carbon in the matrix. The C dissolved in the matrix undergoes a strengthening reaction, which greatly enhances the ability of C to diffuse outward from the ferrite matrix, resulting in the precipitation of a large number of fine and dispersed alloy carbides. At this time, the size of the ferrite grains decreases sharply. The length of the sample is also reduced accordingly. Combining with Table 2, it can be seen that the precipitation expansion rate of alloy carbides is significantly higher than that of other stages in the IV stage of the vacuum tempering process, which further indicates that the volume change of 700L steel is the most significant in the precipitation stage of alloy carbides.

(1) The tempering process of 700L steel includes 5 stages; cementite I precipitation (50-200 ℃), residual austenite decomposition (200-300 ℃), cementite II precipitation (250-400 ℃), alloy Carbide precipitation (450-580 ℃) and Mn partition (580-650 ℃).

(2) During the vacuum tempering process of 700L steel, the precipitation expansion rate and volume change of stage IV (alloy carbide precipitation) were significantly higher than other stages, which may be an important stage affecting the evolution of residual stress in 700L steel.

(3) Carbide precipitation is the main reaction in the vacuum tempering process of 700L steel. At a heating rate of 2 °C/min, there are obvious hardness peaks in the three stages of cementite precipitation, alloy carbide precipitation and Mn distribution.

Selection of vacuum tempering equipment: In addition to good process design, the selection of vacuum heat treatment equipment is also an important factor in completing the process. The RVT vacuum tempering furnace produced by SIMUWU is an excellent choice for this type of process. Its process performance can fully meet the needs of such thermal processing, with good temperature control accuracy, temperature uniformity and tempering uniformity. High process repeatability, stable production, quality output can be guaranteed.